System of light beam communication



May 3, 1955 H. MUELLER SYSTEM OF LIGHT BEAM COMMUNICATION Filed June21', 1949 5 Sheets-Sheet l Modulating Device l I 3 9 Analyzer I 7 DeviceSignal Source I 2 J 3 5 1 1 /MaduIat/'ngDev/'ce F] Amplifier PolarizingDevice and Detector I max. Intensity Am ift/er 59 @174 Detector M d t e11 a or Ose/l/atar an 0 //er Amp/tf/er SM .and Phase W Sh/ffer AttorneysMay 3, 1955 H. MUELLER 2,707,749

SYSTEM OF LIGHT BEAM COMMUNICATION Filed June 21, I949 3 Sheets-Sheet 2Output Signal Avera e Intensify 2 2. a '1'. J93; law-32".

Phase Shift Doubled Carr/er Modular/an Input Signal Carrier CarrierIntensify I M0 du/af/an Oufpuf Slgna/ Phase Sir/ff 3 Natural L lghfCarr/er Modular/an F I g. 2

Input Slgna/ ln van/or Hans Mueller by mmuaud A Horney;

May 3, 1955 H. MUELLER SYSTEM OF LIGHT BEAM COMMUNICATION Filed June 21,1949 5 Sheets-Sheet 3 .219 a a d Fig. 8.

Modulator Oscillator Ma du/afor Oscf/lafar In venfor Hans Mueller y m a!m Attorneys United States SYSTEM OF L'KGHT BEAM CQIVZR EUNECATEGN HansMueller, Belmont, Mass., assignor of one-half to Robert H. Rimes,Belmont, Mass.

Application lane 21, 1949, Serial No. 1Gtl,36tl

34 Claims. (Cl. 256-7) The present invention relates to communicationand more particularly to the transmission and reception of signalintelligence with the aid of electromagnetic waves.

An object of the invention is to provide a new and improved system forsecret signaling.

Another object is to provide a new and improved system for signaltransmission and reception.

A further object is to provide a new and improved system for lightmodulation.

In the copending application of Hans Mueller and Robert H. Rines, SerialNo. 1,002, filed January 7, 1948, issued December 23, 1952, as UnitedStates Letters Patent No. 2,623,165, there is disclosed a novellight-modulation method and system for producing large light effects atultrasonic frequencies substantially independent of frequency and withhigh signal-to-noise ratio.

A further object of the present invention is to provide a new andimproved system employing the light-modulation principles described inthe said copending application.

Other and further objects will be explained hereinafter and will be moreparticularly pointed out in the appended claims.

In summary, the present invention involves producing a beam ofelectromagnetic waves having at least adjacent first and secondpolarized portions. portions are preferably complementarilyplane-polarized. The first polarized portion of the beam is passedthrough a first transparent medium or medium portion and the secondpolarized portion of the beam is passed through a second transparentmedium or medium portion adjacent the first medium or medium portion.The signal to be transmitted is produced, and a first component of thesignal is fed to the first medium or medium portion to alter the stateof polarization of the first portion of the beam emerging from the firstmedium or medium tion of the beam emerging from the second medium or Imedium portion. The first and second beam portions emerging from therespective first and second media or medium portions are then directedalong a common direction incoherently to superpose the beam portions andto form a partially polarized beam comprising a naturalelectromagnetic-wave component and a signal-varying resultant polarizedcomponent of the superposed said altered states of polarization of thefirst and second portions of the beam of electromagnetic waves. Inaccordance with a preferred embodiment of the invention, the media ormedium portions are rendered birefringent by vibrational waves, adjacentmedia or medium portions being controlled by adjacent half-wavelengthsor out-of-phase components of the vibrational-wave signal. A pluralityof alternate medium portions may thus produce, in response to thevibrations in the medium, elliptically polarized waves from thecorresponding incident alternate The adjacent beam "ice plane-polarizedlight beam portions, and adjacent alternate medium portions may producecomplementary elliptically polarized waves from the correspondingcomplementary plane-polarized adjacent alternate incident beam portions.The resultant elliptically polarized component of the partiallyelliptically polarized beam produced by the incoherent superposition ofthe above-described elliptically polarized waves may then be received.By converting the resultant elliptically polarized component into aplane-polarized wave, the received beam may be analyzed and the signalintelligence detected. Preferred systems for carrying out the presentinvention are hereinafter described in detail.

The invention will now be more fully described in connection with theaccompanying drawings, Fig. 1 of which is a block diagram schematicallyillustrating an apparatus operating in accordance with the broadprinciples underlying the present invention; Figs. 2, 3 and 4 arewave-form diagrams explanatory of the operation of the apparatus of Fig.1; Fig. 5 is a perspective view of a combined transmitting and receivingcommunication system embodying the present invention in preferredapparatus form; Figs. 6 and 7 are explanatory wave-form diagramsillustrating the operation of the system of Fig. 5; Fig. 8 is aschematic side-elevation of a modified transmitter constructed inaccordance with the present invention embodying successive layers ofphase-shifting material; Fig. 9 is a View similar to Fig. 8 of anothermodification embodying a phase-shifting material and a plurality ofdifferently oriented polarizing members; Fig. 10 is a view similar toFig. 8 of a further modification in which a modulating signal isintroduced by mechanically vibrating polarizing members.

In order that the terminology hereinafter employed to describe thevarious states of polarization of electromagnetic waves may beunambiguous and clear, the following definitions are presented.

Natural or unpolarized light, such as light from an ordinary lightsource, is of equal intensity along all directions lying in any planenormal to the direction of propagation of the light. Such light may beconsidered as comprising an electric vector, the amplitude or magnitudeof which varies sinusoidally with time. The electric vector assumes alldirections and phases of oscillation with the same probability. Nomatter what type of analyzer may be rotated in the path of a beam oflight having such a natural state of polarization, no direction will befound in which the light intensity is greater than in any otherdirection.

Plane-polarized light, sometimes called linearly polarized light, may beconsidered as comprising an electric vector, the amplitude of whichsinusoidally varies with time and the direction of which is alwaysparallel to a particular predetermined direction in some plane normal tothe direction of propagation of the light. Such planepolarized light maybe produced, for example, with the aid of Nichol prism, a polaroidsheet, or a similar doubly refracting device inserted in the path of abeam of natural light. The plane formed by the direction of the electricvector and the direction of propagation is often termed the plane ofpolarization. The orientation or polarization of the electric vectordepends upon the angular orientation of the plane-polarizing Nicholprism or similar device. If a second plane-polarizing device, frequentlytermed an analyzer, is rotated in the path of a planepolarized wave, amaximum intensity of light will be received through the analyzer when itis oriented parallel to the direction of orientation of the electricvector of the plane-polarized light. When the analyzer is oriented atright angles to the direction of polarization of the planepolarizedwave, however, the light becomes extinguished. For all otherorientations of the analyzer, more or less light intensity willpenetrate the analyzer depending upon whether its angle of orientationis nearer to parallelism with or normality to the orientation of theelectric vector of the light.

Circularly polarized light may be considered as comprising horizontaland vertical plane-polarized components of equal peak amplitude, andequal light frequency, but phase-displaced ninety degrees apart so thatthe resultant electric vector described a circle. Like natural light,the intensity of circularly polarized light, when viewed through arotating plane-polarizing analyzer, for example, appears the same in alldirections lying in any plane normal to the direction of propagation.circularly polarized light may be differentiated from natural light,however, by employing the combination of a quarterwave phase-shiftingplate, hereinafter described, and a plane-polarizing analyzer. Thequarter-wave plate will relatively shift the phase of the horizontal andvertical plane-polarized components another ninety degrees so that theresultant electric vector becomes once more a plane-polarized electricvector which may, of course,

be extinguished, as before explained, by proper orientation of theplane-polarizing analyzer. No possible combination or orientation of aquarter-wave plate and a plane-polarizing analyzer, however, willproduce extinction of a beam of naturally polarized light. Thequarterwave plate, previously referred to, may comprise a doublyretracting crystal in which the index of refraction presented to onecomponent of an incident polarized light beam is sufficiently differentfrom the index of refraction presented to the complementary component ofthe planepolarized light to cause the two components to be relativelyphase-shifted ninety degrees or a quarter wavelength in passing throughthe plate. When the resultant electric vector of the phase-shiftedcomponents describes a circle by rotating clockwise, the polarization issaid to be right-circularly polarized. When, on the other hand, theelectric vector describes a circle in rotating counterclockwise,left-circularly polarized light is produced.

If the phase shift introduced between complementary plane-polarizedcomponents of an incident light beam is not exactly ninety degrees, buthas a value ranging from between zero degrees and ninety degrees, orbetween ninety degrees and one hundred eighty degrees, and so on,

the resultant electric vector describes an ellipse and not a circle,and, such light is commonly termed elliptically polarized light. If thephase shift is ninety degrees but the amplitudes of the complementaryplane-polarized components are not equal, elliptically polarized lightis also produced. The magnitude of the electric vector and hence theintensity of the light will not be the same in all directions, as in thecase of circularly polarized light, but will be less along the directionof one of the complementary plane-polarized components of the incident 7light, termed the minor axis of the ellipse, than along the direction ofthe other complementary component, termed the major axis of the ellipse.When viewed through a rotating plane-polarizing analyzer, therefore,when the direction of orientation of the plane-polarizing analyzer isparallel to the major axis of the ellipse, a maximum intensity of lightis received. When, on the other hand, the analyzer is oriented parallelto the minor axis of the ellipse, a minimum intensity of light isreceived, but there can be no complete extinction of light in anydirection of orientation of the plane-polarizing analyzer. If acomposite analyzer comprising a quarter-wave plate and aplane-polarizing analyzer is employed, there is a direction oforientation along which complete extinction is possible. This may beunderstood from the following considerations. Since the components ofthe elliptically polarized light along the major and minor axes of theellipse are initially phase-displaced by ninety degrees, they willproduce plane-polarized light when passed through the quarter-waveplate. A plane-polarizing analyzer, po-

sitioned beyond the quarter-wave plate, can thus extinguish the light.If, instead of the quarter-wave plate, a phase plate is employed thatintroduces a phase shift corresponding to the initial phase displacementbetween any other complementary component electric vectors than thoseparallel to the major and minor axes of the ellipse, then the othercomponent electric vectors may be brought into phase to produce aplane-polarized wave. This plane-polarized wave may be extinguished by aproperly oriented plane-polarizing analyzer. In right-ellipticallypolarized light, the resultant electric vector of the component electricvectors parallel to the major and minor axes of the ellipse rotatesclockwise. Left-elliptically polarized light is produced by acounterclockwise rotation of the resultant electric vector in describingthe ellipse. Circularly polarized light, before discussed, is a specialcase of elliptically polarized light in which the eccentricity of theellipse is zero.

When there is a mixture of natural light and some other type ofpolarized light, the mixture is termed partially polarized light. Amixture of natural light and elliptically polarized light, for example,is termed partially elliptically polarized light.

When two light beams have different light frequencies or wavelengths, ordiffering phase-varying relationships, the beams are said to beincoherent. Light beams produced by two different light sources, forexample, are incoherent, since they by necessity have differentfrequency or wavelength distributions and phase relationships. Lightbeams from a common source that travel over vastly different paths aresimilarly incoherent because of incoherent phase relationships. Lightbeams may even be transmitted from a common source along equal paths andmay still be incoherent if one beam, for example, suffers a frequencyshift, such as a Doppler shift, in passing along its path. Incoherentlight beams, unlike coherent light beams, do not interfere to produceresultant effects such as diffraction patterns. Incoherent beams remain,instead, distinct and separate beams, one superimposed upon the other,but not interfering with the one other.

Referring to Fig. 1, adjacent light beams, 51 and 53 are shown passingthrough respective light-modulating devices 1 and 3 which may, forexample, comprise mechanical shutters, Kerr cells, birefringent mediaand the like. The light beams S1 and S3 may each have a particularpredetermined state of polarization. The two beams may comprise adjacentportions of a single light source or may emanate from separate lightsources. The beam S3 is shown passing through a polarizing device 5 ofany desired character before reaching the modulating device 3. Even if,therefore, the beams S1 and S3 originate from the same source, they areincoherent by the time they reach the modulators l and 3.

It has previously been proposed to superpose adjacent incoherent beamssuch as, for example, rightand leftcircularly polarized light beamsintermittently transmitted by shutter or other light-modulation systems.At a receiving station unequipped with a proper analyzer, thecombination of rightand left-circularly polarized light would appear tobe natural light and there would be no hint that signal intelligence wasbeing transmitted along the beams. With the aid of a right or leftquarterwave analyzer that will respectively produce a plus or minusninety degree phase-shift between complementary plane-polarizedcomponents, however, one of the two beams, either the rightorleft-circularly polarized beam, may be converted into a plane-polarizedwave and the intelligence may be obtained therefrom.

In accordance with the present invention, however, a much more secrettransmission system is provided by producing out-of-phase modulationupon adjacent preferably elliptically or circularly polarized beams.

Returning to the simple example of Fig. l, a source of signals, such as,for example, a sine-wave generator 7,

is shown operating the modulating devices 1 and 3. The

source 7 preferably feeds one signal component to operate the modulatingdevice 1 and a preferably out-ofphase signal component to operate themodulating device 3. The modulated light emerging from the device 1 istherefore out of phase with the modulated light emerging from the device3. If the modulator 7 is a sine-wave generator, as before suggested, theintensity I1 of the light beam emerging from the device 1, may at a timei=0, Fig. 2, pass through a minimum value I1 minimum. One-half cyclelater, at time t=t1, the light intensity I1 may have sinusoidally variedto a maximum value 11 maximum. At a later time t=r2, one-half cycle,later, the light intensity I may again reach its minimum value I1minimum. The intensity I3 of the difierently polarized incoherent beamemerging from the modulating device 3, on the other hand, at time t=0,has a maximum value I3 maximum, since the device 3 is operated inanti-phase with the device 1. At time t ri, onehal'r cycle later, thelight intensity I3 passes through a mini- ;mum value I3 minimum. thelight intensity I3 again passes through its maximum value I maximum.Assuming that the light intensities from the source S1 and the source S3that impinge upon the devices 1 and 3 are initially the same, andassuming that the modulators 1 and 3 are identical in structure andperformance, the maximum and minimum values of the emerging lightintensities I and I3 will be the same. 11 maximum Will then equal I3maximum, and I1 minimum will equal 13 minimum.

The two incoherent and diiferently polarized beams of intensity I1 andI3. having the same maximum and minimum intensities, are then directedinto space along a common direction toward, for example, a photocellreceiver or detector 9, a human eye, a camera, a lightsensitive mosaic,or some other light receiver. Since the receiver 9 does notdifferentiate between the different states of polarization of the lightreaching it, it receives the incoherently superposed differentlypolarized beams of intensity I3 and ii. The resultant intensity receivedby the receiver 9, therefore, is a constant value Ir, as shown in Fig.4, since the beams I1 and I rise and fall in intensity in anti-phase.The photocell or other receiver 9 cannot, therefore, detect thesine-wave or other modulating intelligence carried by the beams. Thebeams would appear, rather, as a single constant-intensity beam uponwhich no intelligence is super-imposed.

If, on the other hand, a proper type of analyzer it is inserted beforethe photocell 9 so that the state of polarization of one of the beams I1and 13 may be eliminated, the receiver may receive only the other stateof polarization of the other beam and the modulating signal intelligencemay be recovered in the photocell 9. As a specific example, if theresultant beam In is a partially polarized beam comprising, as anillustration, a natural-light component and an elliptically polarizedcomponent, when this beam is focused upon the photocell 9 by, forexample, a lens 8, the photocell 9 will detect no signal modulation.Even the use of a conventional plane-polarizing analyzer 11, would be ofno avail in detecting the modulation intelligence. Only if a particularcombination of an appropriate phase-shifting plate and a properlyoriented plane-polarizing analyzer is used, that is capable ofselecting, for example, the major axis of the elliptically polarizedcomponent in preference to the other components of the mixed natural andelliptically polarized beam, can the photocell 9 detect the signalmodulation.

A practical system with the aid of which the present invention may bepracticed is illustrated in preferred form in Fig. 5. An oscillator 13is shown connected between the electrodes 15 and 17 of a plurality ofpiezoelectric crystals 19, to vibrate the crystals in response to theoscillations of the oscillator 13. The electrode 15 is shown adjacent apreferably strain-free transparent medium 21 as of glass, fuzed quartz,or any similar transparent material. The vibrations of the crystals 19One-half cycle later, at t=t2,

will cause molecular-vibration waves to be set up in the medium 21. Itis to be understood, of course, that any other vibrating means, such asa magnetostrictive vibrator, a magnetomotive element or any othervibrating device, may equally well be employed.

As described in the said copending application, standing waves will beset up in the medium 21 as the vibration Wave travels to the top of themedium and reflects back down again. There will be nodal regions 23 ofthe medium 21 which remain unaifected by the vibration wave. The mediumportions 25, 27, 29, 31, etc. in between these nodal regions, however,do not remain un affected by the vibrations. .At any one instant oftime, the medium portion 25, for example, may be compressed While theadjacent medium portion 27 is being dilated. The medium portion 2?,similarly to the medium portion 25, will also be at this timecompressed, and the adjacent medium portion 31 will be dilatedcorresponding to the dilation of the medium portion 27. Each mediumportion 25, 27, 29, 31, etc., will pass from compression to dilation attwice the frequency of the vibrational wave propagated into the mediumfrom the crystals 19 as a result of the reflection of the waves from thetop of the medium. The medium portions 25, 27, '29, 31, etc., thus actas separate media forming shutters to t.e light passing thereth-rough asthey change from conditions of compression to conditions of dilation atdouble the frequency of the oscillations of the oscillator 13.

The distance M2 between successive nodal regions 23 of the medium 21, orthe distance between corresponding portions of the medium portions 25,27, 29, 31, etc., corresponds to half the wave-length of the vibrationalwaves propagated into the medium .21 by the crystals 19. There is a timedelay produced between the shutter operation of each successive section.25, 27, 29, 31, etc., that is dependent upon the period betweensuccessive oscillations of the oscillator 13.

As explained in the said copending application, when the medium portions25, 27, 29, 31, etc., become compressed or dilated, they becomebirefringent to polarized light entering the medium 21. A portion of thebeam from a parallel-ray light source 33-53 may pass through aplane-polarizer 35 having an orientation of, for example, forty-fivedegrees with respect to the vertical, and may then penetrate thecorresponding medium portion 25 of the medium 21. As the medium portion25 is compressed by the vibrational waves, the indices of refraction ofthe medium portion 25 for the vertical and horizontal electric vectorcomponents of the forty-five degree incident plane-polarized lightbecome unequal. The horizontal and vertical electric vector componentsbecomes thus phase displaced so that the resultant light emerging fromthe medium portion 25 is, in general, elliptically polarized. Thedifference in the indices of refraction of the medium portion 25presented to the vertical and horizontal electric vector components ofthe incident light by the vibrational straining of the medium, is calledthe birefringence of the medium. It is this birefringence which producesthe phase displacement or shift A between the vertical and horizontalelectric vector components of the incident light as given by theequation where T is the thickness of the medium from the front to itsrear face, in is the index of refraction of the medium presented to thevertically polarized component, F111 is the refractive index presentedto the horizontally polarized component, 7\ is the wavelength of thesound waves or other vibrational waves in the medium 21, and 1r is theratio of the circumference to the diameter of a circle.

As can be seen from the above equation, increased thickness of themedium shutter 21 increases its birefringence-producing efiiciency. Thepower required to obtain a given birefringent effect decreases as themedium thickness increases, the relation being that of the inversesquare, but the larger the medium the more difficult it is tomanufacture it strain-free and the more serious heating and otherundesirable effects.

If the phase shift A has a value between zero and ninety degrees, theforty-five degree polarized light incident upon the medium portion 25will become elliptically polarized, as shown schematically at 45, withthe major axis of the ellipse at a forty-five degree angle with respectto the vertical. The eccentricity of the ellipse depends upon the amountof the phase shift Aqs. When, of course, this phase shift is exactlyninety degrees, the emerging light is circularly polarized.

When, on the other hand, the medium portion 25 becomes dilated by theultrasonic waves, the refractive index lZv again becomes different thanthe index in}, but in the opposite sense from the difference produced bythe compression of the medium 25, so that the phaseshift A has the samemagnitude but the opposite sign. If the birefringence produced bycompression is referred to as positive birefringence, and thephase-shift is re ferred to as a positive phase shift +IA then when themedium is dilated, an equal amount of negative bifringence is producedand an equal phase shift --[A| results. The light beam emerging from thedilated medium portion 25 will again be elliptically polarized, but themajor axis of the ellipse will be in the minus forty-five degreedirection with respect to the vertical, complementary to the major axisof the elliptically polarized waves previously discussed for the case ofcompression.

Neglecting, for the moment, the time difference between the instantsthat each vibration Wave reaches the successive portions of the medium21, at the time that the medium portion 25 is being compressed, themedium portion 29 is also compressed and the medium portion 27 and themedium portion 31 are dilated. The light beams passing through themedium portions 25 and 29 will thus be elliptically polarized as shownat 45 and 49, while the light beams passing through the medium portions27 and 31 will be complementarily elliptically polarized as shown at 47and 51. The alternate medium portions 25 and 29 are thus operated inanti-phase with the adjacent alternate medium portions 27 and 31,outof-phase components of the vibrational signal produced by thecrystals 19, controlling adjacent medium portions. Actually, of course,the light emerging from the various medium portions is rapidly passingfrom a plane-polarized state to various states of elliptically polarizedlight, 7

back to plane-polarized light, and back to other states of ellipticallypolarized light, as the amplitude of the signal vibration-wavesinusoidally increases and decreases to produce its effects upon themedium 21. In general,

however, the light becomes elliptically polarized, and it is convenient,for purposes of explanation, to consider the elliptically polarizedstates 45 and 49 produced by the action of the medium portions 25 and 29on the incident light beam at a particular time during, for example,compression, and the elliptically polarized states 47 and 51 produced ata corresponding time by the action of the medium portions 27 and 31during dilation.

The electric vector describing the elliptically polarized beams 45 or 49produced during compression of the respective medium portions 25 and 29,will rotate in the opposite direction to the direction of rotation ofthe electric vectors describing the complementary elliptically polarizedbeams 47 and 51, produced during dilation of the respective mediumportions 27 and 31. This occurs because the phase shift produced by themedium portions 26 and 29 during compression is of opposite sign to thephase shift produced in the medium portions 27 and 31 during dilation,as before explained.

The light emerging from the medium 21 comprises, therefore, a partiallypolarized beam having a natural wave-component and a resultantelliptically polarized wave-component corresponding to the resultant ofthe elliptically polarized beams 45, 47, 49 and 51. Partial cancellationmay, of course, take place between the adjacent light beams 45, 47, 49and 51-because of the oppositely rotating electric vectors. A principalresultant direction of polarization may thus be produced. Any receivingstation having at least a plane analyzer will thus be informed that thelight beam contains signal intelligence which it may then intercept.

if the plane-polarized light impinged upon the medium portion 25 ispolarized at an angle of plus fortyfive degrees from the vertical by theadjacent polarizer 35, and the light impinged upon the next adjacentmedium portion 27 is polarized at an angle of minus fortyfive degreesfrom the vertical by its adjacent planepolarizer 37, shown oriented atright angles to the polarizer 35, and the next medium portion 29receives light from its adjacent polarizer 39 polarized at plusfortyfive degrees, and the next medium portion 31 receives light fromits adjacent polarizer 41 polarized at minus forty-five degrees, thenthe electric vectors of the elliptically polarized beams 45, 47, 49, 51,etc., emerging from the respective medium portions 25, 27, 29, 31, etc.,will all rotate in the same direction. Again a combination of naturallypolarized light and a different resultant elliptically polarized lightis formed. Since, however, the electric vector of the light emergingfrom all the medium portions is at any time always rotating in the samedirection, there is no cancellation of the light intensity emerging fromsuccessive medium portions, as in the previously discussed case wherethe electric vectors of alternate medium portions rotate in oppositedirections.

The use of the successive strips of differently oriented polarizers 35,37, 39, 41, etc., furthermore, which will be hereinafter referred to asa stripped polarized, produces more efficient operation than can beproduced with a system of the type described in the said copendingapplication in which each medium portion of the medium 21 receives lightof the same polarization. The reasons for this improved efliciency willbe subsequently explained.

The beams of elliptically polarized waves emerging from the mediumportions 25, 27, 29, 31, etc., are directed along a common direction,indicated by the arrow in Fig. 5, by means of the directing action of aparabolic reflector 53. The reflector 53 will direct substantiallyparallel rays of light from the source 33 through the medium 21. Afilter 55 may be used if it is desired to transmit monochromaticelectromagnetic waves. If an infra-red ray filter 55 is employed, thetransmitted light waves will, of course, be invisible to the eye. Thisis desirable in secret signaling applications.

The cumulative light beam formed by the adjacent incoherent beamsemerging from the medium portions 25', 27, 29, 3.1, etc., appearsindistinguishable from natural light at a. distant station. Theincoherent superposition of these beams produces, as before mentioned, apartially polarized beam comprising, in general, a natural-lightcomponent and an elliptically polarized component that is the resultantof the superposition or mixture of the elliptically polarized waves 45,47, 49, 51, etc., emerging from the adjacent medium portions 25, 27, 29,31, etc. If, for example, a photocell 9 at the focus of a parabolic jlight-receiving reflector 57, receives this cumulative beam,

it will merely indicate a constant intensity of light in an amplifier orother circuit connected thereto, without indicating the presence of anymodulation. The photocell 9 may, of course, be replaced by any otherlight detector such, for example, as a light-sensitive mosaic, or aphotographic film of either the stationary or moving-picture type, butstill no modulation or intelligence will be detected. if an analyzer,such as the plane-polarizing analyzer 58, is inserted before thephotocell or other detector 9, furthermore, no possible orientation ofthe plane-polarizing analyzer 58 will provide detection of a principaldirection in which the light intensity is greatest, and it is thusimpossible to tistinguish the received light beam from an unmodulatedconstant-intensity light beam of natural light. No one receiving thelight beam with conventional equipment, would even know that polarizedlight was being employed, and the presence of signal intelligence in thelight beam would not be detected.

If, however, an appropriate birefringent medium or phase-shifting plateti is used in conjunction with the plane-polarizer analyzer to comprisea composite appropriately oriented analyz r, and if the phase-shiftingplate so produces a sufiicient phase shift between the complementaryelectric vector components of the resultant elliptically polarized-wavecomponent of the received partially elliptically polarized beam to bringback into phase or into antiphase the electric vector componentsparallel to and normal to the major axis of the el iptically polarizedresultant wave, then, and then only, will the waves emerging from thephase shifter 6% be planepolarized so that they may be analyzed by theplanepola-rizing analyzer 53. At this time, assuming that the photocell9 is sensitive to the light-beam frequency and that its amplifier systemis tuned to the carrier frequency of the oscillator 13, the modulationsignal superimposed thereon may be detected.

For a particular medium 21 having a particular thickness T and aparticular amplitude of compressional wave, the phase shift introducedby the periodic birefringence of the medium in response to theultrasonic waves may produce circularly polarized light emerging fromthe medium portions 25, 2'7, 29, Si, etc. The rotation of the electricvector of the circularly polarized light emerging from the variousportions 25', 27, 29, 31, etc. of the medium 31 will always rotatetogether in a common direction if the complementary polarizers 35, 37,etc. are employed as above described. The electric vectors will rotatefirst clockwise, then counterclockwise, then clockwise again in responseto the positive and negative birefringence periodically produced at thefrequency of the transmitted and reflected vibrational waves in themedium Zll. Similarly, in the general case where an ellipticallypolarized component of the mixture of natural and elliptically polarizedlight is produced, the electric vector will rotate first clockwise, thencounterclockwise, then clockwise, again, at the radio-frequency of theoscillator 13. The transmitted beam thus appears indistinguishable fromnatural light at conventional lightreceiving stations.

Unless, therefore, the receiving station is equipped first, with aparticular com osite analyzer comprising a phase-shifting plate 6i) ofproper phase-shifting properties and a plane-polarizing analyzer 53appropriately relatively orientated; second, with a photocell 9 ofcharacteristics such that it responds to the wavelength of the lightwave; and third, with an amplifier system tuned to the carrier frequencyof the oscillator 13, it is not possible to detect that polar zation oflight is being employed, let alone to detect the signal intelligencebeing transmitted along the light beam. Extremely secure signalling maythus be effected.

The oscillator 13 preferably comprises an ultrasonic or radio-frequencyoscillator for causing the piezoelectric crystals 19 to produce standingwaves of high frequency in the medium 21. l have successfully employedoscillating frequencies ranging from audio frequencies up to andincluding high radio frequencies. The standing waves produced by tieoscillator 13 driving the crystals 19, moreover, as discussed in thesaid copending application, may be employed as a carrier-frequency waveand in this connection may be modulated by a modulator 59 such as asource of audio or video signals, or a source of pulses for keying theoscillator on and off, or any other modulating signal source.

I have, for example, constructed and successfully operated systems ofthe type illustrated in Fig. 5 employing square media 21 of opticalcrown glass and pyrex glass six inches long, six inches wide and twoinches thick. Such media require about 0.1 watt of driving energy percubic inch of volume to operate satisfactorily as lightmodulationshutters. The opposite surfaces of the media were parallel to 0.001inch. A similar medium of fused quartz was found to yield greaterbirefringent effects and hence more intense results than the opticalglass with 10 to 50 times lower driving power requirement because of lowacoustic damping, but it was difficult to maintain uniform nodalportions 23 when the ultrasound waves were propagated into the medium21, and sharp resonance points of the shutter rendered it satisfactoryonly for dotdash communication and not for audio or video modulation.Two parallel rows of six X-cut quartz crystals of crystal dimensions oneby one by 0.283 inch were attached to one of the six-inch by two-inchsurfaces of the medium 21 by soldering their upper electrodes to themedium. The common electrode 15 of Fig. 5, for example, may be sosecured to the bottom surface of the medium 21. These crystals weredriven by a conventional well-shielded Hallicrafter crystal oscillatorcircuit 13 a a radio-frequency of about 0.4 megacycle. A large number ofmechanical resonant frequencies of the medium 21 can be found, usuallyabout 10 kilocycles apart. For the previously described media 21, asharp set of nodes and a maximum birefringence effect was produced forthe mechanical resonance resulting at 0.4 megacycle vibrationalfrequency. Vibrational waves of this frequency produced in the medium 21about nine adjacent periodically compressed and dilated portions 2-5,27, 29, 31, etc. Successive strips of polaroid 35, 37, 39, 41, etc., onecorresponding to each respective medium portion 25, 27, 29, 31, etc.,and each oriented to planepolarize light at right angles to the plane ofpolarization of the next adjacent strip, where placed adjacent themedium 21. The width of each strip was substantially the same as thewidth of each of the medium portions, which, in turn, was substantiallythe same as the half wavelength of the standing waves produced in themedium. The alternate polarizing strips 35 and 39 were placed adjacentthe alternate indium portions 25 and 29; the adjacent alternate strips37 and 41 were placed near the adjacent alternate medium portions 27 and31; and so on.

The birefringence produced by longitudinal strains set up in the medium21 by the ultrasonic vibrations was employed to polarize ellipticallythe incident li ht waves, by polarizing the incident waves alongdirections plus or minus forty-five degrees from the vertical, as beforediscussed. When the polarizing strips 35, 3f, etc. were oriented at plusforty-live degrees, with respect to the vertical or the direction ofpropagation of the vibra tional waves, and the strips 37, 41, etc. wereoriented at minus forty-five degrees, the before-described results wereobtained in response to the birefringent action of these longitudinalstrains. The birefringence produced by transverse stresses, also set upin the medium 21 by the vibrations, was also employed by polarizing theincident light vertically or horizontally. The alternate polarizingstrips 37, 41, etc. were, for example, oriented vertically while thealternate intermediate strips 3:1, 39, etc. were oriented horizontallyto employ the birefringence produced by the transverse stresses. Thesetransverse modes of vibration, however, have been found less preferablefor the purposes of the present invention than the longitudinalvibrational modes because of poor vibrational patterns in the medium 21and because of relatively narrow signal band widths obtainabletherewith. Orientations of the polarizing strips between fortyiivedegrees and the vertical or horizontal produced results partaking of theno. ture of both transverse-and longitudinal-mode birefringence, thetransverse-node effect predominating the nearer the orientation to thevertical or horizontal, and

the longitudinal-mode effect predominating closer to plus or minusforty-five degrees.

An automatic-frequency control was incorporated at the transmitter byemploying a feedback crystal 2 similar to the crystals 19 at the topsurface of the medium 21 opposite the driving crystals 19. A pluralityof feedback crystals may, if desired, be employed. The vibrationstransmitted through the medium are received by and will vibrate thecrystal 2 and the resulting oscillations may be amplified in anamplifier 4 and fed back, for example, betwen the grid and cathode ofthe oscillator tube of the crystal oscillator 13 to control thefrequency of the oscillator 13. The shutter medium 21, therefore,automatically controls the frequency of the oscillator 13 as smallshifts occur in the mechanical vibration or resonance frequency of theshutter resulting from temperature changes during operation. Theamplifier 4 preferably contains a conventional phase-shifter to insureproper phase feedback from the crystal, as is well known in the art.

Employing the before-described two-inch thick medium 21 of optical glassabout six to eight inches in front of a 450-watt airplane landinglamp-reflector system 33-53, having a beam width of about 7 to 10degrees, an infrared filter 55 having a peak sensitivity of from between8000 and 10,000 Angstroms, an infra-red-light-type polaroid strippedpolarizer 35, 37, 39, 41, etc., I successfully transmitted and receivedspeech modulation over distances of up to seven miles. At the receivingend, I employed thin cellophane sheets 60, which are substantiallyquarter-Wave plates for light of 9000 Angstroms wavelength, aninfra-red-type polaroid plane-analyzer 58, a Farnsworth type 6?EAsix-stage photo-tube multiplier 9 having substantial sensitivity forwavelengths of the order of 10,000 Angstroms and a maximum sensitivityat about 8000 Angstroms, and an associated receiving circuit responsiveto the 0.4 megacycle radiofrequency carrier of the oscillator 13 andprovided with a detector for detecting the modulation superimposed onthe carrier.

In the systems described in the said copending application the receivingsystem, even if it comprises a quarter-wave plate 60 in addition to theplane-polarizing analyzer 58, produces a signal at twice the frequencyof the carrier-wave propagated into the medium 21 by the oscillator 13.The photocell receiver circuit must thus be tuned either to themodulation envelope of the carrier wave or to the second harmonic of thefrequency of the oscillator 13. In accordance with the principles of thepresent invention, however, the receiver can be tuned only to thefrequency of the oscillator 13, as will now be explained.

The intensity I of the light received through the analyzer 58 by thephotocell 9 depends upon the square of the sine of half the phase shiftproduced by the irefringence in the medium 21. This relationship betweenintensity I and phase-shift 4) is in part plotted in curve A of Fig. 6.The system disclosed in the said copending application operates so thateach portion of the medium 21 between nodes, such as the portion 25, forexample, goes through compression strain, zero strain, and dilationstrain, twice each cycle of the vibrational waves propagated into themedium, because the medium responds to both the directly propagated andreflected Waves. The light penetrating the analyzer 58, therefore,becomes completely extinguished twice for each cycle of the sound waves.This is equivalent, therefore, to operating from a point 0 on the curveA, Fig. 6, first swinging positively up on the right-hand side of curveA from point 0 to point P, then swinging back through point 0 to point-P on the left-hand side of the curve A. Consider, therefore, an inputsignal consisting of a carrier wave and a modulation signal superimposedthereon, fed to the medium 21 by the vibrating crystal 19. As showngraphically in Fig. 6, the modulated carrier oscillates between limitsof P and -P of the curve A as each section of the medium 21 periodicallybecomes compressed and dilated in response thereto. The output signal,appearing as an intensity-modulated light beam, is also plotted in Fig.6, varying between the horizontal base line drawn through the point 0and the parallel horizontal line drawn through the points P and --P. Themodulation envelope is shown produced as an output signal with, however,double the carrier frequency supporting the modulation, as abovedescribed. The shading indicates that each of the double carrierimpulses produces, in general, elliptically polarized light beams fromadjacent portions of the medium, the electric vectors of adjacent beamsrotating in opposite directions. The average light intensity that may bedetected with a plane analyzer and a photocell receiver is shown indotted lines, labelled Average intensity.

With the system of the present invention, on the other hand, operationis effected, not about point 0, but about a point 0 on, for example, theright-hand portion of the intensity-phase-shift curve A plotted in Fig.7. In effect, the present system is optically biased so that the lightintensity does not periodically pass through a zero point as in Fig. 6,but passes from point 0' upward to a point Q and downward through 0 to apoint R, all on one side of the zero point 0. In this manner there isnever any complete extinction produced by the analyzing system 58-60,but, as shown, the modulated carrier-wave input signal operates betweenpoints R and Q to produce a modulated output carrier signal of the samefrequency, and not double the frequency of the input carrier signal. Theshading of the carrier in the output signal indicates the presence ofelliptically polarized light. In the first cycle all of the portions ofthe medium have electric vectors rotating, for example, clockwise, asshown by the clockwise curved arrow, and in the next cycle all of theelliptically polarized light rotates counter-clockwise, as indicated bythe counter-clockwise curve arrow, and so on. The average intensity ofthe output signal, however, is not low, as in the case of the system ofthe copending application, the performance of which is shown in Fig. 6.The average intensity is, on the contrary, high. There is, moreover, aconstant presence of a natural light component in the output signal,labelled Natural light and shown cross-hatched. By means of operating atan optically biased point 0, furthermore, on the substantially linearportion of the intensity-phase-shift curve A, a greatly increasedamplification or gain is produced over the signal gain of the systemsthat operate, as shown in Fig. 6. The combination in the output signalof the natural light component and the elliptically polarized resultantlight component, the direction of rotation of the resultant electricvector of which periodically varies from clockwise to counterclockwiseat the frequency of the carrier wave produced by the oscillator 13, isin itself indistinguishable from natural light without specialequipment, as before described, and permits great security because ofthe large number of factors that must necessarily coincide to detect themodulated elliptically polarized resultant component.

If security is not desired, of course, the phase-shifting plate 60 maybe employed at the transmitter side near the medium 21 instead of in thereceiver. The stripped polarizers 35, 37, 39, 41, etc., may, if desired,furthermore, be applied to the medium as by cement or any other means.

If desired, moreover, as shown in Fig. 10, the oscillator 13 maycontinuously oscillate the crystal 19 with a continuous-wave carrier.The modulation of that carrier may be effected by vibrating the strippedpolarizers. The stripped polarizers 35, 37, 39, 41, etc. may be carriedby a holder 62, which is yieldingly mounted between supports 64, so thatin their normal positions,

the polarizers are adjacent the nodes 23 in the medium 21. A portion ofthe support 62, shown at 66, may be of ferromagnetic material in orderthat, in response to the modulation signal of the modulator 59, asmanifest in a coil 5'8 wound about the portion 66, the carrier 62 may beperiodically moved up and down periodically to bring the strippolarizers 35, 37, 39, 4-1, etc., in front of the respective mediumportions 25, 27, 29, 31, etc. In this manner the modulation signal isapplied by means of the vibrating of the stripped polarizers, thecarrier signal being applied by the oscillator 13, driving the crystals19 to vibrate the medium 21.

Again, where secrecy is of less importance, an optically biased systemmay be provided by employing a single constant polarizer 65, Fig. 8, forpolarizing all the incident light impinging on the medium 21 in the sameplane of polarization. Adjacent the medium 21, however, are strippedphase-shifting plates shown, for example, as quarter-wave plates 75, 77,79 and 81. Assuming that the thickness of the medium 21 is such thatsubstantially circularly polarized waves emerge from the medium, theplate 75, for example, may produce a positive ninety-degree phase shiftin the circularly polarized light emerging from its adjacent mediumportion 25; the strip of phase-shifting material 77, on the other hand,adjacent the medium portion 27 will produce a negative ninety-degreephase shift; the phase-shifting plate 79, adjacent the medium portion29, may produce a positive ninety-degree phase shift and the strip 81,adjacent the medium portion 31, a negative ninety-degree phase shift.The array of phase-shifting plates 75', 77, 79, 81, etc.,

acteristic, Fig. 7, is produced and light comprising a natural-lightcomponent and, in general, a resultant elliptically polarized componentresulting from the incoherent superposition of the ellipticallypolarized waves emerging from adjacent portions of the medium shutter 21are produced.

Alternately, the medium 21 may itself be initially stressed as byputting it under a constant pressure from a vise, not shown, between,for example, its top and bottom surfaces so that, in its initial state,even in the absence of vibrations produced by the crystals 19, themedium constitutes a birefringent half-wave phase-shifter in and ofitself. Similarly, piezoelectric and other crystals having a permanentberefringent stress may also be employed.

The system of Fig. 5, in which a stripped polarizer 35, 37, 39, 41,etc., is used in conjunction with a uniform phase-shifting plate 60,will hereinafter be called system (a). The system of Fig. 8, in which auniform polarizer 66 is employed in conjunction with a strippedphaseshifting plate 75, 77, 79, 81, etc., will hereinafter be calledsystem (b). Assume that the birefringence produced in the medium 21 issuificient to produce a ninetydegree phase shift between horizontal andvertical components of incident plane-polarized light. A comparison ofthe results then produced by systems (a) and (b) for incident light ofdifferent types of polarization when the systems are quiescent, as when,for example, the oscillator 13 is inoperative, and when the systems areoperative, as when the ultrasound waves are propagated therein, follows:

Plane Polarized Circularly Polarized Elliptically Polarized IncidentLight System System System System System System Quiescent OperatingQuiescent Operating Quiescent Operating System (a) Natural- PartiallyCircular. Natural Partially Circulan Natural Partially Elliptical.System (1)) Plane..." Partially Plane do Partially Planem. PartiallyPlunc Partially Plane.

may comprise alternate right and left quartz quarterwave plates, and maybe placed either between the plane polarizer 66 and the medium 21 ordirectly after the medium 21, as shown in Fig. 8. But in either case itis preferably positioned very close to the medium 21 in order to obtainclose correspondence between the phaseshifting plates and the respectivemedium portions of the medium 21.

Again, if security in transmission is not of the prime essence,optically biased performance may be obtained by positioning between theplane polarizer 66 and the medium 21 a phase-shifting plate such as, forexample, a quarter-wave plate 68, Fig. 9, for a medium 21 of suchthickness as to produce substantially a ninety-degree birefringent phaseshift. In this manner, circularly polarized light impinges upon all ofthe sections in the medium 21. Circularly polarized light penetrates themedium 21 and becomes plane-polarized therein as a result of theninety-degree phase shifts produced by the medium. The strips ofpolarizers 35, 37, 39, 41, etc., similar to those used in connectionwith the embodiment of Fig. 5, may then be placed to receive theemerging plane-polarized waves from their respective correspondingregions 25, 27, 29, 31, etc., of the medium 21. The plate 68 need not bea quarter-wave plate if the medium produces less than or more thansubstantially a ninety-degree phase shift, but, in general, will producea phase shift corresponding to the phase shift produced by the mediumwhen birefringent. It is to be understood that even if the phase shiftproduced in the medium does not exactly equal the phase shift of thesystem 66-68, elliptically polarized waves of large eccentricity, almostplane-polarized, will result which can be almost extinguished by thepolarizers 35, 37, 39, 41, etc.

In all cases, however, a biased operation on the substantially linearportion of the intensity-phase-shift char- I have shown boththeoretically and in practice that system (:2) provides a more eflicientsystem than system (b). System (a), moreover, provides secrecy which isunobtainable with system (b) since the use of merely a plane polarizerwill detect that the transmission from system (b) involves thepolarization of light. This may be seen from the above table. System (b)will always produce partially plane-polarized light, when ultrasoundWaves are propagated in the medium 21, whereas system (a) producespartially circular or partially elliptically polarized light. If,furthermore, the phase-shifting plate employed in system (b) is notexactly of the right design, an observer with a plane polarizer willactually be able to observe dot-dash communication if it is beingemployed, or with the aid of a photocell and audio amplifier, willdetect voice or other modulation intelligence. In system ((1), however,the periodically alternating right and left elliptically polarizedbeams, emerging from the medium portions 25, 27, 29, 31, etc., form apartially elliptically polarized beam that is practicallyindistinguishable from natural light. An observer requires not only avery accurate proper phase plate and polarizing analyzer and a properradio-frequency tuned photocell receiver, but also a proper relativeorientation of the phase plate and polarizing analyzer in order todetect any intensity fluctuations or to find out that the transmissioneven involves the polarization of light.

Systems constructed in accordance with the present invention areparticularly insensitive to jamming or interference. This is true sincethe use of the proper phase plate 6%) permits distinguishing betweeneven much stronger interfering natural, plane or other polarized lightand the periodically changing left and right elliptically or circularlypolarized light emerging from the medium The medium 21 may be vibratedin compressional,

transversal, flexural, torsional and other modes depend ing upon thecoupling of the crystals or other vibrators to the medium, the degree ofhomogeneity of the medium, and the transverse and longitudinalreflections of the Waves from the boundaries of the medium, as well asother factors.

If the incident light rays pass obliquely through the medium 21,penetrating adjacent layers or portions of the medium, they Will emergewith a phase shift that is a resultant of the phase shifts produced bythe positive and negative birefringence of the medium portions throughwhich the rays pass. On integration, the intensities of some obliquerays will therefore cancel the intensities of other rays so that thelight intensity will increase and decrease with angular position of theaxis of the transmitter, passing through several maxima and minima.Instead of reducing the angular aperture of the light beam to overcomethis effect, it is desirable to reduce the sound-vibration frequency tokeep the size of the layers as large as possible, and to place thestripped polarizers of Figs. 5, 9 and 10 and the stripped phase platesof Fig. 8 as close to the medium shutter 21 as possible. If, however, itis desired to obtain a larger angular aperture well-known spreadinglenses, one corresponding to each portion 25, 27, 29, 31, etc., may beplaced between the light source 33 and the corresponding medium portionsto converge the light rays to a focus within the corresponding portions25, 27, 29, 31, etc. of the medium, the rays then diverging from eachmedium portion into beams of Wider angular aperture.

For any given frequency, the stripped shutter system cannot thus be usedfor as large an angular aperture as the uniform shutter disclosed in thesaid copending application. The stripped shutter system of the presentinvention, furthermore, while producing more efficiency and greatersecrecy, requires special stripped polarizing or phase-shifting elementsfor each carrier frequency.

Further modifications will occur to those skilled in the art and allsuch are considered to fall within the spirit and scope of the presentinvention as defined in the appended claims.

What is claimed is:

1. An apparatus for signal transmission that comprises means forproducing a beam of electromagnetic light Waves having adjacent firstand second portions, means for polarizing the first portion of the beam,means for simultaneously polarizing the second portion of the beam,means for passing the first polarized portion of the beam through afirst light transparent medium, means for passing the second polarizedportion of the beam through a second light transparent medium adjacentthe first medium, means for rendering the first and second portions ofthe beam incoherent, means for producing the signal to be transmitted,means for rendering the first medium birefringent in response to a firstcomponent of the signal thereby to alter the state of polarization ofthe first portion of the beam emerging from the first medium and forrendering the second medium birefringent in response to a secondcomponent of the signal thereby to alter the state of polarization ofthe second portion of the beam emerging from the second medium, andmeans for directing the incoherent first and second portions of the beamemerging from the respective first and second media along a commondirection incoherently to superpose the beam portions and thereby toform a partially polarized beam comprising a naturalelectromagnetic-Wave component and a signalvarying resultant polarizedcomponent of the superposed said altered states of polarization of thefirst and second portions of the beam of electromagnetic Waves.

2. An apparatus for signal transmission that comprises means forproducing a beam of electromagnetic light Waves having adjacent firstand second portions, means for plane-polarizing the first portion of thebeam along a predetermined plane, means for plane-polarizing the secondportion of the beam along a different predetermined plane to render thesecond beam portion incoherent with the first plane-polarized portion ofthe beam, means for passing the first plane-polarized portion of thebeam through a first light transparent medium, means for passing thesecond plane-polarized portion of the beam through a second lighttransparent medium adjacent the first medium, means for producing thesignal to be transmitted, means for feeding a first component of thesignal to the first medium to render the first medium birefringentthereby elliptically to polarize the first portion of the beam emergingfrom the first medium and for feeding a second component of the signalto the second medium to render the second medium birefringent, therebyelliptically to polarize the second portion of the beam emerging fromthe second medium, and means for directing the incoherent first andsecond elliptically-polarized beam portions emerging from the respectivefirst and second media along a common direction incoherently tosuperpose the beam portions and thereby to form a partially ellipticallypolarized beam comprising a natural electromagnetic-Wave component and asignal-varying resultant elliptically polarized component of thesuperposed said elliptically polarized first and second portions of thebeam of electromagnetic waves.

3. An apparatus for signal transmission that comprises means forproducing a beam of electromagnetic light Waves having adjacent firstand second portions, means for plane-polarizing the first portion of thebeam along a predetermined plane, means for plane-polarizing the secondportion of the beam along a different predetermined plane to render thesecond beam portion incoherent with the first plane-polarized portion ofthe beam, means for passing the first plane-polarized portion of thebeam through a first light transparent medium, means for passing thesecond plane-polarized portion of the beam through a second lighttransparent medium adjacent the first medium, means for producing thesignal to be transmitted, means for feeding a first component of thesignal to the first medium to render the first medium birefringentthereby circularly to polarize the first portion of the beam emergingfrom the first medium and for feeding a second component of the signalto the second medium to render the second medium birefringent, therebycircularly to polarize the second portion of the beam emerging from thesecond medium, and means for directing the incoherent first and secondcircularly-polarized beam portions emerging from the respective firstand second media along a common direction incoherently to superpose thebeam portions and thereby to form a partially circularly polarized beamcomprising a natural electromagnetic-Wave component and a signal-varyingresultant circularly polarized component of the superposed saidcircularly polarized first and second portions of the beam ofelectromagnetic waves.

4. An apparatus for signal transmission that comprises means forproducing a beam of electromagnetic light waves having adjacent firstand second portions of substantially the same intensity, means forplane-polarizing the first portion of the beam along a predeterminedplane, means for plane-polarizing the second portion of the beam along aplane substantially perpendicular to the said predetermined plane,thereby rendering the first and second polarized beam portionsincoherent, means for passing the first plane-polarized portion of thebeam through a first light transparent medium, means for passing thesecond plane-polarized portion of the beam through a second lighttransparent medium similar to and adjacent the first medium, means forproducing the signal to be transmitted, feeding a first component of thesignal to the first medium to render the first medium birefringentthereby elliptically to polarize the first portion of the 17 beamemerging from the first medium with the major axis of the ellipsedisposed along a predetermined direction and for feeding a secondcomponent of' the signal subtantially identical with but out of phasewith the first signal component to the second medium to render thesecond medium birefringent thereby elliptically to polarize the secondportion of the beam emerging from the second medium with the major axisof the ellipse substantially perpendicular to the said predetermineddirection, and means for directing the incoherent first and secondelliptically polarized beam portions emerging from the respective firstand second media along a common direction incoherently to superpose thebeam portions and thereby to form a partially elliptically polarizedbeam comprising a natural electromagnetic-wave component and asignal-varying elliptically polarized resultant component of thesuperposed elliptically polarized first and second portions of the beamof electromagnetic waves.

5. The apparatus described in claim 4 and in which the firstplane-polarizing means is oriented so that the first portion of the beamof electromagnetic Waves is plane-polarized at forty-five degrees withrespect to a dimension of the firstmedium.

6. The apparatus described in claim 4 and in which thefirstplane-polarizing means is oriented so that the first portion of thebeam of electromagnetic waves is plane-polarized along a planesubstantially parallel to a dimension of the first medium.

7. An apparatus for signal transmission that comprises means forproducing a beam of electromagnetic light waves having adjacent firstand second portions of substantially the same intensity, means forplane-polarizing the first portion of the beam along a predeterminedplane, means for plane-polarizing the second portion of the beam along aplane substantially perpendicular to the said predetermined plane,thereby rendering the first and second polarized beam portionsincoherent, means for passing the first plane-polarized portion'of thebeam through a first light transparent medium, means for passing thesecond plane-polarized portion of the beam through a second lighttransparent medium similar to and adjacent the first medium, means forproducing the signal to be transmitted, means for feeding a firstcomponent of the signal to the first medium to render the first mediumbirefringent thereby circularly to polarize the first portion of thebeam emerging from the first medium and for feeding a second componentof the signal substantially identical with but out of phase with thefirst signal component to the second medium to render the second mediumbirefringent thereby circularly to polarize the second portion of thebeam emerging from the second medium, and means for directing theincoherent first and second circularly polarized beam portions emergingfrom the respective first and second media along a common directionincoherently to superpose the beam portions and thereby to form apartially circularly polarized beam comprising a naturalelectromagnetic-wave component and a signal-varying circularly polarizedresultant component of the superposed circularly polarized first andsecond potrions of the beam of electro-magnetic waves.

8. An apparatus for signal communication that comprises means forproducing a beam of electromagnetic light waves having adjacent firstand second portions of substantially the same intensity, means forplane-polarizing the first portion of the beam along a predeterminedplane, means for plane-polarizing the second portion of the beam along aplane substantially perpendicular to the said predetermined planethereby rendering the first and second polarized beam portionsincoherent, means for passing the first plane-polarized portion of thebeam through a first light transparent medium, means for passing thesecond plane-polarized portion of the beam through a second lighttransparent medium similar to and adjacent the first medium, means forproducing the signal to be transmitted, means for feeding a first component of the signal to the first medium to-render the first mediumbirefringent thereby elliptically to polarize the .first portion of thebeam emerging from the first medium with the major axis of the ellipsedisposedalong a predetermined direction and for feeding a secondcomponent of the signal substantially identical with but out of phasewith the first signal component to the second medium to render thesecond medium birefringent thereby elliptically to polarize the secondportion of the beam emerging from the second medium with the'major axisof the ellipse substantially perpendicular to the said predetermineddirection, means for directing the incoherent first and secondelliptically polarized beam portions emerging from the respective firstand second media along a common direction incoherently to superpose thebeam portions and thereby to form a partially elliptically polarizedbeam comprising a natural electromagnetic-wave component and asignal-varyingelliptically polarized resultant component of thesuperposed elliptically polarized first and second portions of the beamof electromagnetic waves, means for analyzing the signal-varyingelliptically polarized resultant component of the partially ellipticallypolarized beam and means for detecting the signal from the analyzedbeam.

9. An apparatus for signal communication that comprises means forproducing a beam of electromagnetic light waves having adjacent firstand second portions of substantially the same intensity, means forplane-polarizing the first portion of the beam along a predeterminedplane, means for plane-polarizing the'second portion of the beam along aplane substantiallyperpendicular to the said predetermined plane,thereby-rendering thefirst and second polarized beam portionsincoherent, means for'passing thefirst plane-polarized portion of thebeam through a first light transparent medium, means for passing thesecond" plane-polarized portion of the beam through a second lighttransparent medium similarto and adjacent the first medium, means'forproducing a periodic signal to be transmitted comprising a carrierwaveof predetermined frequency modulated by a-modulating signal, meansfor producing birefringence in the first medium in response to a firstcomponent of the periodic signal'to render the first mediumbirefringent, thereby periodically elliptically to polarize the firstportion of the beam emerging from the first medium with the majoraxis ofthe ellipse disposed along a predetermined direction, and for producingbirefringence in the second medium in responseto a second component-ofthe periodic signal one-hundred eightydegrees phase-displaced fromthefirst periodic signal component to render the second mediumbirefringent, thereby periodically elliptically to polarize 'the' secondportion'of-the'beam emerging from the second medium'with the major axisof the ellipse substantially perpendicular tothe said predetermineddirection,' means for directing the incoherent first and secondelliptically polarized beam portions emerging from the respectivefirst'and second media along a'common direction incoherently tosuperpose'the beam portions and to form a partially ellipticallypolarized beam comprising a' natural electromagnetic-wave component anda signal-varying elliptically polarized resultant component of thesuperposed elliptically"polarized first and second portions or the beamof electromagnetic waves, means for analyzing the signal-varyingelliptically polarized resultant component of the partially ellipticallypola'rized'beam, means for receiving and-amplifying the modulatedcarrier-frequency'signal contained 'in the analyzed beam and means forreproducing-the modulating signal. l V

"10.Ari' apparatus for signal transmission that comprises means for'producing'abeam of electromagnetic light waves having adjacent first-andsecond portions of substantially the same intensity, means forplane-polarizing the first portion of the beam-alonga predeterminedplane, means for plane-polarizingthe second portion of the beam along aplane substantially perpendicular to the said predetermined planethereby rendering the first and second polarized beam portionsincoherent, means for passing the first plane-polarized portion of thebeam through a first portion of a light transparent medium, means forpassing the second plane-polarized portion of the beam through a secondportion of the light transparent medium adjacent the first portion ofthe medium, means for producing the signal to be transmitted, means forpropagating the signal as a vibrational wave into the transparent mediumto render the first and second portions of the medium birefringent inresponse to the action of successive oppositely-phased components of thevibration wave thereby elliptically to polarize the first and secondportions of the beam emerging from the respective first and secondportions of the medium with the major axis of the ellipse describing thefirst elliptically polarized beam portion substantially perpendicular tothe major axis of the ellipse describing the second ellipticallypolarized beam portion, and means for directing the incoherent first andsecond elliptically polarized beam portions emerging from the respectivefirst and second portions of the medium along a common directionincoherently to superpose the beam portions and thereby to form apartially elliptically polarized beam comprising a naturalelectromagnetic-wave component and a signalvarying ellipticallypolarized resultant component of the superposed elliptically polarizedfirst and second portions of the beam of electromagnetic waves.

11. An apparatus for signal communication that comprises means forproducing a beam of electromagnetic light waves having adjacent firstand second portions of substantially the same intensity, means forplane-polarizing the first portion of the beam along a predeterminedplane, means for plane-polarizing the second portion of the beam along aplane substantially perpendicular to the said predetermined plane,thereby rendering the first and second polarized beam portionsincoherent, means for passing the first plane-polarized portion of thebeam through a first light transparent medium, means for passing thesecond plane-polarized portion of the beam through a second lighttransparent medium similar to and adjacent the first medium, means forproducing a periodic signal to be transmitted comprising a carrierwaveof predetermined frequency modulated by a modulating signal, means forproducing birefringence in the first medium in response to a firstcomponent of the periodic signal to render the first mediumbirefringent, thereby periodically elliptically to polarize the firstportion of the beam emerging from the first medium with the major axisof the ellipse disposed along a predetermined direction and forproducing birefringence in the second medium in response to a secondcomponent of the periodic signal one-hundred eighty degrees phase-dis.

placed from the first periodic signal component to render the secondmedium birefringent, thereby periodically elliptically to polarize thesecond portion of the beam emerging from the second medium with themajor axis of the ellipse substantially perpendicular to the saidpredetermined direction, means for directing the incoherent first andsecond elliptically polarized beam portions emerging from the respectivefirst and second media along a common direction incoherently tosuperpose the beam portions and to form a partially ellipticallypolarized beam comprising a natural electromagnetic-wave 12. Anapparatus for signal transmission that comprises means for producing abeam of electromagnetic light Waves having a plurality of successivelydisposed portions of substantially the same intensity, means forplane-polarizing the alternate portions of the beam along apredetermined plane, means for plane-polarizing the remaining portionsof the beam along a plane substantially perpendicular to the saidpredetermined plane, thereby rendering the successively disposedpolarized beam portions incoherent, means for passing the alternateplanepolarized portions of the beam through corresponding alternateportions of a light transparent medium, means for passing the remainingplane-polarized portions of the beam through corresponding remainingportions of the transparent medium, means for producing the signal to betransmitted, means for propagating the signal as a vibrational wave intothe transparent medium to render the portions of the medium birefringentin response to the action of successive oppositely phased components ofthe vibration wave thereby elliptically to polarize the alternate andthe remaining portions of the beam emerging from the respectivealternate and remaining portions of the medium with the major axis ofthe ellipse describing the alternate elliptically polarized beamportions substantially perpendicular to the major axis of the ellipsedescribing the remaining elliptically polarized beam portions, and meansfor directing the incoherent elliptically polarized beam portionsemerging from the respective portions of the medium along a commondirection incoherently to superpose the beam portions and thereby toform a partially elliptically polarized beam comprising a naturalelectromagnetic-wave component and a signalvarying ellipticallypolarized resultant component of the superposed elliptically polarizedportions of the beam of electromagnetic waves.

13. In a system having a medium that is transparent to light along apredetermined direction and that, when molecularly vibrated at apredetermined frequency to produce standing waves therein, becomesbirefringent to the light passing therethrough along the predetermineddirection, communication apparatus that comprises means for passingthrough a plurality of successively disposed portions of the medium,each of width substantially equal to the half wavelength of the standingwaves, along the predetermined direction a beam of light having aplurality of successively disposed portions respectively correspondingto the plurality of successively disposed half-wavelength portions ofthe medium, means for polarizing in a predetermined state ofpolarization the light in alternate beam portions prior to its passagethrough the corresponding alternate portions of the medium, means forpolarizing in a state of polarization differing by an oddintegermultiple of ninety degrees from the said predetermined state ofpolarization the light in the remaining beam portions prior to itspassage through corresponding remaining portions of the medium, meansfor molecularly vibrating the medium at the predetermined frequency,

means for modulating the molecular vibrations of the medium inaccordance with a signal, means for analyzing the light transmittedthrough the medium and means for detecting the variations in theanalyzed light resulting from the signal modulations.

14. The apparatus of claim 13 and in which the molecular vibration ofthe medium is effected by means for propagating ultrasonic waves intothe medium.

15. A transmitting system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined frequency to produce standingwaves therein, becomes birefringent to the light passing therethroughalong the predetermined direction, means for passing through a pluralityof successively disposed portions of the medium, each of which issubstantially equal to the vhalfwavelength of the standing waves, alongthe predetermined direction a beam of light having a plurality ofsuccessively disposed portions corresponding to the plurality ofsuccessively disposed half-wavelength portions of the medium,

a pluralityof similar polarizers, one disposed in each alternate beamportion neareachcorresponding alternate portion of the medium and havingsubstantially'the same width as the medium portion for similarlypolarizing the light prior to its passage through the alternate mediumportions, a further plurality of "similar polarizers, each disposed topolarize at an angle diifering by an odd-integer multiple of ninetydegrees from the polarization of the first-named plurality of similarpolarizers and one disposed in each of the remaining beam portion neareach corresponding remaining portion of the medium and havingsubstantially the same width as the medium portion for similarlycomplementarily polarizing the light prior to its passage through theremaining medium portions, and means for molecularly vibrating themedium at the predetermined frequency to render the successivelydisposed medium portions birefringent in anti-phase.

16. Apparatus as set forth in claim 15 the vibrating means of whichcomprises means for propagating ultrasonic waves into the medium.

17. Apparatus as set forth in claim 15 the vibrating means of Whichcomprises a plurality of piezoelectric means.

l8. Apparatus as set forth in claim 15 in which the polarizers disposedin successively disposed beam portions near corresponding successivelydisposed medium portions are plane-polarizers oriented, respectively, atplus and minus forty-five degrees with respect to the direction ofpropagation of the molecular vibrations. l9. Apparatus as set forth inclaim 15 in which the polarizersdisposed in successively disposed beamportions near corresponding successively disposed medium portions areplane-polarizers oriented," respectively, parallel or normal to thedirection of propagation of the molecular vibrations.

' 20. A communication system having, in combination, a medium that istransparent to light along a predeter mined direction and that, whenmolecularly vibrated at a predetermined frequency to produce standingWaves therein, becomes birefringent to the light passing therethroughalong the predetermined direction, means for passing through a pluralityof successively disposed portions of the medium, each'of which issubstantially equal to the half-wavelength of the standing Waves, alongthe predetermined direction a beam of light having a plurality ofsuccessively disposed portions corresponding to the plurality ofsuccessively disposed half-wavelength portions 'of the medium, aplurality of similar polarizers, one disposed in each alternatebeamportion near each corresponding alternate portion of the medium andhaving substantially the same width as the medium portion for similarlypolarizing the light prior to its passage through the alternate mediumportions, a further plurality of similar polarizers, each disposed topolarize at an angle diifering by an oddinteger multiple of ninetydegrees from the polarization of the first-named plurality of similarpolar izers and one disposed in each remaining beam portion near eachcorresponding remaining portion of the medium and having substantiallythe same width as the medium portion for similarly complementarilypolarizing the light prior to its passage through the remaining mediumportions, and means for molecularly vibrating the medium at thepredetermined frequencyto render the successively disposed mediumportions birefringent in anti-phase, means for varying the molecularvibrator in accordance with a signal, means for receiving the lightpassed through the medium, means for converting the received light intoplane-polarized light, means for analyzing the converted plane-polarizedlight, and means for detecting the variations in the analyzed light toreproduce the signal.

21. A transmitting system having, in combination, a medium that istransparent to light along a predetermined directiomand that, whenmolecularly vibrated at a predetermined frequency to produce standingwaves therein, becomes birefringent to the lightpassin'g therethrough "aalong the predetermined direction, means for passing through a pluralityof successively disposed portions ofthe medium, each of widthsubstantially equal to the half-wavelength of the standing waves, alongthe predetermined direction a beam of light having a pluralityof successively disposed portions corresponding to the plurality of successivelydisposed half-wave portions of the medium, a plurality of similarpolarizers one disposed in each alternate beam portion near eachcorresponding alternate portion of the medium and having substantiallythe same width as the medium portion for similarly polarizing the lightprior to its passage through the alternate medium portions, a furtherplurality of similar polarizers, each disposed to polarize at an anglediffering by an odd-integer multiple of ninety degrees from thepolarization of the first-named plurality of similar polarizers and onedisposed in each remaining beam portion near each correspondingremaining portion of the medium and having substantially the same widthas the medium portion for similarly complementarily polarizing the lightprior to its passage through the remaining medium portions, means formolecularly vibrating the medium at the predetermined frequency therebyto render the successively disposed medium portions birefringent inanti-phase and to form complementary elliptically polarized beams havinga common direction of electric vector rotation emerging fromsuccessively disposed medium portions, and means for directing theemerging beams along a common direction incoherently to superpose thebeams and to form a partially elliptically polarized beam comprising anatural light-wave component and an elliptically polarized resultantcomponent, the electric vector of which reverses direction of rotationat the said predetermined frequency.

22. A communication system having, in combination, a medium that istransparent to light along a predetermined direction, and that, whenmolecularly vibrated at a predetermined frequency to produce standingwaves therein, becomes birefringent to the light passing therethroughalong the predetermined direction, means for passing through a pluralityof successively disposed portions of the medium, each of widthsubstantially equal to the half- Wavelength of the standing Waves, alongthe predetermined direction a beam of light having a plurality ofsuccessively disposed portions corresponding to the plurality ofsuccessively disposed half-wave portions of the medium, a plurality ofsimilar polarizers one disposed in each alternate beam portion near eachcorresponding alternate portion of the medium and having substantiallythe same width as the medium portion for similarly polarizing the lightprior to its passage through the alternate medium portions, a furtherplurality of similar polarizers, each disposed to polarize at an anglediffering by an odd-integer multiple of ninety degrees from thepolarization of the first-named plurality of similar polarizers and onedisposed in each remaining beam portion near each correspondingremaining portion of the medium and having substantially the same Widthas the medium portion for similarly complementarily polarizing the lightprior to its passage through the remaining medium portions, means formolecularlyvibrating the medium at the predetermined frequency therebyto render the successively disposed medium portions birefringent inanti-phase and to form complementary elliptically polarized beams havinga common direction of electric vector rotation emerging fromsuccessively disposed medium portions, means for directing the emergingbeams along a common direction incoherently to superpose the beams andto from a partially elliptically polarized beam comprising a naturallightwave and an elliptically polarized resultant component the electricvector of which reverses direction of rotation at the saidpredeterminedfrequency, means for receiving the directed partiallyelliptically polarized beam, means for phase-shifting the complementaryplane-polarized components of the elliptically polarized resultantcomponent of the received beam sufficient to produce plane-polarizedlight, means for analyzing the planepolarized light, and means tuned tothe said predetermined frequency for receiving the analyzed light.

23. A communication system as claimed in claim 22 in which theeccentricity of the ellipses of the elliptically polarized waves issubstantially zero and the phase-shifting means comprises a quarter-waveplate.

24. In a system having a medium that is transparent to light along apredetermined direction and that, when molecularly vibrated at apredetermined frequency to produce standing waves therein, becomesbirefringent to the light passing therethrough along the predetermineddirection, communication apparatus that comprises means for passingthrough a plurality of successively disposed portions of the medium,each of width substantially equal to the half wavelength of the standingwaves, along the predetermined direction a beam of light having aplurality of successively disposed portions respectively correspondingto the plurality of successively disposed half- I wavelength portions ofthe medium, means for similarly polarizing the light in alternate beamportions, means for similarly polarizing the light in the remaining beamportions but with a different polarization than that of the saidalternate beam portions, means for molecularly vibrating the medium atthe predetermined frequency, means for modulating the molecularvibrations of the medium in accordance with a signal, means foranalyzing the light transmitted through the medium and means fordetecting the variations in the analyzed light resulting from the signalmodulations.

25. In a system having a medium that is transparent to light along apredetermined direction and that, when molecularly vibrated at apredetermined frequency to produce standing waves therein, becomesbirefringent to the light passing therethrough along the predetermineddirection, transmitter apparatus that comprises means for passingthrough a plurality of successively disposed portions of the medium,each of width substantially equal to the half wavelength of the standingwaves, along the predetermined direction a beam of light having aplurality of succesively disposed portions respectively corresponding tothe plurality of successively disposed halfwavelength portions of themedium, means for similarly polarizing the light in alternate beamportions, means for similarly polarizing the light in the remaining beamportions but with a different polarization than that of the saidalternate beam portions, means for molecularly vibrating the medium atthe predetermined frequency, and

means for modulating the molecular vibrations of the medium inaccordance with a signal.

26. In a system having a medium that is transparent to light along apredetermined direction and that, when molecularly vibrated at apredetermined frequency to produce standing waves therein, becomesbirefringent to the light passing therethrough along the predetermineddirection, transmitter apparatus that comprises means for passingthrough a plurality of successively disposed portions of the medium,each of width substantially equal to the half wavelength of the standingwaves, along the predetermined direction a beam of light having aplurality of successively disposed portions respectively correspondingto the plurality of successively disposed half-wavelength portions ofthe medium, means for similarly polarizing the light in alternate beamportions, means for similarly polarizing the light in the remaining beamportions but with a complementary polarization to that of the saidalternate beam portions, means for molecularly vibrating the medium atthe predetermined frequency, and means for modulating the molecularvibrations of the medium in accordance with a signal.

27. A transmitting system having, in combination, a

medium that is transparent to light along a predetermined direction, andthat, when molecularly vibrated at a predetermined frequency to producestanding waves therein,

becomes birefringent to the light passing therethrough along thepredetermined direction, means for passing through a plurality ofsuccessively disposed portions of the medium, each of widthsubstantially equal to the half-wavelength of the standing waves, alongthe predetermined direction a beam of light having a plurality ofsuccessively disposed portions corresponding to the plurality ofsuccessively disposed half-wave portions of the medium, a plurality ofsimilar polarizers one disposed in each alternate beam portion near eachcorresponding alternate portion of the medium and having substantiallythe width of the medium portion for similarly polarizing the alternatebeam portions, a further plurality of similar polarizers, each disposedto polarize at an angle differing by an odd-integer multiple of ninetydegrees from the polarization of the first-named plurality of similarpolarizers and one disposed in each remaining beam portion near eachcorresponding remaining portion of the medium and having substantiallythe width of the medium portion for similarly complementarily polarizingthe remaining beam portions, means for molecularly vibrating the mediumat the predetermined frequency thereby to render the successivelydisposed medium portions birefringent in anti-phase and to formcomplementary elliptically polarized beams having a common direction ofelectric vector rotation emerging from successively disposed mediumportions, and means for directing the emerging beams along a commondirection incoherently to superpose the beams and to form a partiallyelliptically polarized beam comprising a natural light-wave componentand an elliptically polarized resultant component, the electric vectorof which reverses direction of rotation at the said predeterminedfrequency.

28. An apparatus as claimed in claim 1 and in which the said means forrendering the first and second portions of the beam incoherent isdisposed between the said beamproducing means and the saidlight-transparent media.

29. An apparatus as claimed in claim 1 and in which the said means forrendering the first and second portions of the beam incoherent isdisposed in the path of the said first and second portions of the beamemerging from the said first and second media.

30. An apparatus as claimed in claim 1 and in which the said polarizingmeans are plane-polarizing means and the said means for rendering thefirst and second portions of the beam incoherent comprises first andsecond phaseshifting plates, one disposed in the path of each of thesaid first and second portions of the beam emerging from the said firstand second media, the phase-shifting plates producing phase shifts ofopposite sign.

31. An apparatus as claimed in claim 30 and in which the phase-shiftingplates comprises right and left quarterwave plates, and the said firstand second media, when birefringent, produce substantially ninety-degreephase Shifts.

32. An apparatus as claimed in claim 1 and in which the said polarizingmeans are means for producing elliptically polarized light; the saidfirst and second media are of such thickness that, when renderedbirefringent, they convert the light emerging therefrom intosubstantially plane-polarized light; and the said means for renderingthe first and second portions of the beam incoherent comprises first andsecond plane polarizers, one disposed in the path of each of the saidfirst and second portions of the beam emerging from the said first andsecond media, the plane polarizers producing polarizations that differby an odd multiple of ninety degrees.

33. An apparatus as claimed in claim 32 and in which the means forproducing elliptically polarized light is adjusted so that theeccentricity of the ellipse of the elliptically polarized light issubstantially zero, and the said media are of such thickness that, whenrendered birefringent, they produce substantially a ninety-degreephase-shift.

34. Apparatus as claimed in claim 10 and in which the saidplane-polarizing means are vibrated periodically 25 into and out ofalignment with the said portions of the medium that are renderedbirefringent, thereby to modulate the said superposed beam portions inaccordance with the vibration of the said plane-polarizing means.

26 Nicolson Nov. 30, 1937 Salinger June 6, 1944 Hammond July 16, 1946Young Sept. 3, 1946 ODea Oct. 25, 1949 Shamos et al. Nov. 28, 1950 RinesDec. 23, 1952 Mueller et al. Dec. 23, 1952 FOREIGN PATENTS Great BritainSept. 23, 1919 Great Britain Aug. 12, 1936

